1. Field of the Invention
The present invention relates to an injection molding process for molten plastic or resin.
2. Prior Art
A conventional process for injection molding is well known in the art. A plastic pellet is fed into a hopper of a injection molding machine such as an extruder. In the extruder, the plastic pellet is carried in a cylinder having a piston or a screw carrier and heated to produce a molten plastic. The molten plastic is extruded via a nozzle into a die assembly. The molten plastic is then maintained or held in the die assembly to provide a mold product. There are two types of pressures at the cylinder and the die assembly. A plunger pressure, that is, an injection pressure is created on the piston or screw in the cylinder while an internal resin pressure for molten plastic occurs in the die assembly.
FIG. 1 shows a graph of the plunger pressure of the piston or screw of the conventional extruder vs. the internal resin pressure within a cavity defined in the die assembly when a molding is produced according to the conventional injection process. As shown in FIG. 1, the plunger pressure of the extruder is increased to a maximum ram pressure upon filling the molten resin to the die assembly. Upon holding and cooling the molten plastic the plunger pressure is decreased to the intermediate pressure and then to a minimum ram pressure. It should be noted that the internal resin pressure according to conventional molding process has a synchronous pressure variation to that of the plunger pressure. The internal resin pressure is therefore increased upon filling the molten resin to the die assembly, with substantially following a trace of the plunger pressure. The internal resin pressure then gradually is decreased upon holding and cooling the molten resin in the die assembly since the molten resin is gradually cooled and solidified with a slight shrinkage of the resin. Such pressure change is considered to result from various factors such as, for example, the plunger pressure generated by a hydraulic pressure system, the volume or capacity of the cavity in the mold assembly, the flow resistance of molten resin against a passage from the nozzle to the cavity, a diameter of a gate facing to the cavity and the like. The most effective factor to be considered is that a volume of the cavity to be extruded does not change upon filling and holding the molten plastic in the die assembly by the predetermined mold clamping force. A filling time is also effective to prevent the mold surface under cooling from flashing.
An extruded area EA of the molten resin with an internal resin pressure IRP to a die assembly having a fixed mold clamping force MCF has an upper limit defined by: EQU EA.times.IRP=MCFmax
If the value of the EA.times.IRP, that is, the MCFmax exceeds the mold clamping force of the die assembly, as shown in FIG. 5, a space or gap 3 between surfaces 1a and 2a of respective male and female dies 1 and 2 is provided with an opening amount h. The molten resin M therefore enters the gap 3 to cause a flash Ma to occur on a molded article. This would result in poor products and sometimes in interruption of the molding. On the other hand, in order to increase a filling density of the molded article to a value higher than a predetermined value, it is required to increase the internal resin pressure to a value higher than a predetermined value and, after all, the extruded area of the molten resin is limited by the mold clamping force of the die assembly.